Research On The Technology Of Parametric Sub-bottom Profiling

Parametric transmitting array can transmit a low frequency, narrow beam and low side-lobe sound waves in a smaller size. It is a good choice to use the parametric transmitting array for sub-bottom profiling when the size of the transducer is restricted to be small.Based on nonlinear acoustic principle, this paper designs the parametric transmitting array to detect the buried pipeline.After discussing the forming mechanism of difference frequency wave, sound pressure expression, gain expression and the expression of directivity are derived. When a broadband parametric array used, it is concluded that the sound pressure of difference frequency wave is proportional to the second order partial derivative of the value which is the square of the envelope function. The difference frequency wave characteristics of the different envelope function are simulated in the paper, and lay the foundation for system design.In the system hardware design part, five functional boards are introduced. The source board is designed to provide electricity to the system. Test results show that the source board meets the demand of DC voltage for other circuits; The transmitter board is designed to generate the special signals by the MCU.The signal driven by the drive circuit flows into class D power amplifier circuit, and is finally provided to the transducer. The energy storage capacitors apply enough energy to transmit the signal. Test results show that sound level of the primary frequency wave is 225.6d B, the output power of 720.4W; The receiver board consists of the fixed gain amplifier, the variable gain amplifier and the passive filter. Test results show that the output of the variable gain amplifier is between-8d B and 72 d B, the output of the fixed gain amplifier is 20.8d B; This paper designs DSP minimum system, and use ADC peripherals for the echo signal sampling. An expanded memory is connected through XINTF interface. The DSP communicates with the upper machine by the LAN9128 I which is also connected to the DSP through XINTF. The transmitting synchronous control signal and the gain control signal of the variable gain amplifier are provided by the DSP. System hardware design can meet the needs of the parametric sub-bottom profiling.In the software design chapter, the process how parametric sub-bottom profiler realizes its functions and the signal processing are mainly introduced. The core algorithm modules in the software are explained in detail, such as band-pass FIR filters, envelope detection, extracting and compressing, the bottom of the detection, TVG compensation and so on. The parametric sub-bottom profiler can be normally operatedThe characteristics of the difference frequency signal are tested with the parametric sub-bottom profiler in the paper. Test results show that the primary frequency sound level is 226 d B, difference frequency source level 169.9 d B, the primary frequency wave-3d B angle is 2.9°, the difference frequency wave-3d B angle is 3.5°, broadening appropriately, the difference frequency conversion efficiency is 0.16%, the difference frequency gain 56.1 d B, consistent with theory.